When the valve guides are badly worn it's time to replace them and this is a routine overhaul job. That said I am increasingly of the view that very high precision with valve guide manufacture and fitting is imperative. Valve guides are getting smaller and smaller (some 16v models have 10-11mm OD guides and 5mm valve stems. I am going to say from the outset that I am not a fan at all of guide inserts. I cannot see the sense in boring out a thin-walled guide and putting an insert down it. That just weakens an already weak thing. Nuff said on that.

A new valve to guide running fit might be anything from 0.0005" to 0.002" but checking how loose/worn used ones are is very much down to experience. Having OE data helps, because they will cite a limit. Often the best way is compare a new valve and guide with an old pair, waggle the valve with the valve at 1cm lift and see how loose it is. Worn guides lead to excessive oil consumption, and tip-rock damage to bucket, and the more worn they are the more cast iron guides are to fracture around the stem seal region. In the extreme, a badly worn guide can allow the valve to hit the edge of the piston valve relief.

Remember the valve stem must be in good condition or you are wasting your time with new guides. A worn stem will damage a new guide very quickly. Check the stems with a digital micrometer and compare with OE data. You should polish the old stems in a drill or lathe if using old valves using fine Scotchbrite. A low power drill is safest. Mind you don't get your fingers trapped.

Removal
Guides are best removed by pressing - in either direction. This can be more difficult than it sounds. It certainly helps some if - before commencing, you decarbonise and hot wash the head and soak the old guides in penetrating oil. You also must use a press (with at least 2 ton press capacity) and a robust jig set up at the right angle. Pressing the guides off centre - or worse - knocking them out with an air hammer or club hammer - can cause damage to the guide bore that will show later when you regrind the valve seats. Which you must do - new guides always throw the valve off centre relative to the seat. A well-pressed guide should come out perfectly intact and leave little by way of damage to the guide bore.

Thick guides like 8V/16V Fiat/Lancia models (14/15mm nominal OD) are no problem to press out from the combustion chamber end. Some models like the thin-wall 11mm OD guides in the Vauxhall 16v have a rather fragile bullet-nose and will just fracture if you try to press them from the cc end, and so I always press from the seal end having machined off the seal region first. Ford Zetec guides are such a loose fit they come out easily on their own. Models like Rover 220 are fantastically tight and I have a special rig for doing them.

New guides & sizes
Regarding the effect of bore-guide fit:
1. The guide bore does not alter significantly from the as-bored factory condition, despite having had a guide pressed into it and having run for years. During factory fit, the guide bore swells within its elastic limit and generates hoop stress, after removal of guide it closes up again. In other words the guide bore - although fractionally out-of-round here and there - will accept a new guide of the same outer diameter (OD) as it had before. Certainly on the 1st overhaul anyway, at later overhauls you really need to check the bore for deviation as pressing can tear metal out of the guide bore.
2. Having got the old guides out measure each one and look out for any oversize. If you have a bore gauge measure the bores in the head and compare the two to examine the fit. It will normally be about 0.002". Fiat for example quote 2-4thou" for guide-head fits, myself I'd rather be nearer 2 thou.
3. Compare the new guides with the old ones. I might typically produce my own race guides with a slightly bigger OD than the originals, say +0.001 because honing them out if they are tight in situ is much more satisfactory than a loose fit. A tight fitting guide will tend to close up and you might not be able to get the valve thru it when fitted (even if it was fine prior to that), this is usual.
4. Be aware the many manufacturers produce oversize guides. These are not meant to be pressed straight into the head, they are for heads where the guide bore (in the head) has been rebored to suit. If you press a guide in that is much more than 0.001-0.002" bigger than the original you can easily split the head open. And remember that bronze material guides will deform and accommodate tightness much more readily than cast iron ones. Moreover the tighter the guide the more likely you are to induce damage to guide during fitting..

New Guides - what to look for
1. Check that the guides are same size or say 0.0005-0.001" bigger than the old ones, no bigger. If the guides are too big you can carefully polish them down yourself in a lathe or press-drill using 220 grade tape wrapped on a flat file, but you must be consistent in removal or you'll generate a taper. It is vital to survey the diameter with a digital micrometer as you go. Thinking they are fine when in fact there is no consistency in diameter or parallel might is great until you come to the seat grinding op and the things are miles out of concentricity with the seat. If you can find a machine shop who can regrind them between centres (accurately) then that's great. But I stress 'if'....
2. New guides (and I never fit cast iron, just too brittle for racing) should be dimensionally checked unless they are OE. OE are never wrong in the sense that they will certainly be up to the job. If you just buy aftermarket guides (and there are some horrors on sale these days) and fit them without checking you only have yourself to blame if the new setup is just as bad as the old one (or worse, and yes, I have see it). In terms of checks I measure:
Valve to guide fit; OD - all over - it can vary on poorly produced guides; variance on ID end to end; OD and bore finish; parallelism; stem seal fit; nose shape - can it be improved?

A word or two on race guide bores and finish.
Bronze guides are NOT easy to make. The surface finish all over should be 2.0 Ra or better. Rough guides are no use to anyone. The best have a ground OD and a precision bored or drilled/precision reamed ID. It's very difficult to precision bore much smaller than 8mm because the boring tool gets jammed with swarf and can shear off. And the longer the guide the worse the problem.
With all turning (machining) ops there is a tendency for the tool to 'climb up' on the material (leading to significant variations on OD that can upset concentricity and fit) and so a properly constituted engineering drawing coupled with detailed discussion with the machinist is essential. I prefer a smooth bore and to do my own in-situ honing with FlexHone brushes. This gives me the opportunity to do 'selective fits' with my valves in the even that one valve is slightly different diameter to another, and allows me to generate an 'oil retentive' finish (as with an engine cylinder) that greatly prolongs the life of the guides. It also allows me to relieve any tight spots in the guides after fitting.

Fitting
Prior to fitting the new guides you must put a small radius on the entry of the guide bore of the head and lightly dress up the bore with a split fork deburrer in a drill with 220-400 grade tape - or you're going to get really bad tearing of the new guide with consequential damage in the bore and misalignment. Then wash out the bore with brake cleaner and a bottle-brush and airline. The better you pressed out the old guides the less debris there is going to be in the head but it is usual to have some scoring damage, especially if you pressed from the cc end and the guide was heavily carboned up. With some heads, like the TC Fiat 8V I beadblast first, but I wouldn't recommend that op on 16v heads, you are never going to get them clean afterwards.
The head need to be set up accurately with the valve axis vertical. If you press at angle the guide_is_going-to-be-damaged, or at least thrown off centre. The press tool must sit on a strong region of the guide and I have many, many tools to suit different engines.
I much prefer to press COLD because I can feel the fit. I have given up using nitrogen except with heads that have circlips on the guides because you cannot feel the fit. Fitting on new heads where the guide bores are perfectly round, well, nitrogen is great, but on old heads I would not recommend it.

I lubricate the guide bores in the head with copper-based anti-seize lubricant. If you press dry, you'll tear the bores to bits. Some use oil. I don't, as a rule, use adhesive of any kind, and if you have a problem fit - too loose - you can press right out again and put a very light single knurl on the guide upper section adding no more than ½ thou" to the diameter near the stem seal end to restore the fit. With knurling there is always a risk of throwing the valve axis way out (so far out that you cannot get the seat to clean up) so I stress just a 'light' knurl. What happens on the next overhaul is anyone's guess, have only had to knurl once and that was because someone had rammed massively VW cast iron oversize guides into a nice old 1438 TC head - I imagine you might have to get the head rebored.

After fitting
The first thing is to offer up the valves and get the best pair matched up. Then number_the_valves! Before you forget. If you just mark with a marker pen you are sure to wash it off. Never force a valve into the guide, it can get totally stuck.
If the guides are tight reaming might be necessary, but the hand reamer must be near new ie: sharp and that op is best be done by hand. If you use a drill, be extremely progressive, use low speed and cutting oil or you can easily end up off-centre and mis-shape the internal bore of the new guide, or worse, get the reamer stuck.
Try the valve in the guide from both ends before deciding how to tackle a tight region. If it's tight both ends, ream. If it's just tight in the middle, get a 6/7/8mm dia 180 or 240 grade guide FlexHone and dwell in that region. 180 grit and engine oil will open a guide very quickly at 500-1000rpm and 240 grit is for final cross-hatch finish for oil retention. After reaming or honing you MUST wash out the guide bore with brake cleaner and a brush or you're going to score the valve stems.
In the UK you can get Flex-Hones from Pacehigh (sole appointed UK distributors, honing experts and good friends of mine, tel: 01707 327788) The BRM website is http://www.brushresearch.com .

Remember after final checks, the seat MUST be recut or reground and the honing dust/debris in the bore must be cleaned out with a small 'bottle brush' and brake cleaner or lapping and grinding pilots are going to embed that into the soft bronze material and wear out the valve stem and guide very quickly in service.

Attachments

The HS head sits accurately with valve axis vertical and a case hardened mandrel presses the guides out from the cc end.

HS guide press.JPG (18.3 KiB) Viewed 14000 times

Having got the guides out I record all their features.

HS guide dims.JPG (18.16 KiB) Viewed 14000 times

Precision guides for the GC overhaul of the Vauxhall HS 2.3 produced EXACTLY to drawing.

NE HSR new guides.JPG (21.14 KiB) Viewed 13991 times

Fitting guides is always from the stem seal end and you need a mandrel that sits on a robust region of the guide. Press on the end of the bore and it will collapse instantly.

NE HSR guides fit.JPG (22.38 KiB) Viewed 13982 times

First thing after fitting is to see how well the valve performs in the new guide. It should be a snug fit and just fall under its own weight. If it's tight it can be lightly honed.

NE HSR guide fit test.JPG (26.6 KiB) Viewed 13977 times

Here I am developing a 45 degree cross-hatch honed finish with an 8mm 240 grade FlexHone (powered by a battery operated drill). I use engine oil to hone. I can also 'dwell' to relieve tight spots.

At left - mandrels for guide removal, very tight guides need a case-hardened tool or it will just bend. At right, examples of fitting tools

tools for pressing.jpg (116.56 KiB) Viewed 14086 times

This is a tool for pressing Vauxhall XE guides that have no shoulder. The depth of the recess sets the guide height in the head - that MUST allow for safe full cam lift and fitment of spring seats, stem seals.

press tool no shoulder.jpg (114.55 KiB) Viewed 14046 times

Fiat 8V press tool, guide has a shoulder so you want to press on that strong region. Unfortunately shouldered guides are being phased out.

press tool with shoulder.jpg (113.01 KiB) Viewed 14067 times

HOT TIP. do your porting while the guides are out, or you'll end up with a 'right old knife_and_fork' job in the ports.

A 022.jpg (467.32 KiB) Viewed 14045 times

Polishing old valve stems with fine carborundun and Scotchbrite is a good thing; often means the difference between GO/NO GO in the new guide, but survey the stem dia first with micrometer..